3-ISOPROPYLMALATE DEHYDROGENASE FROM CHEMOLITHOAUTOTROPH THIOBACILLUS-FERROOXIDANS - DNA-SEQUENCE, ENZYME-PURIFICATION, AND CHARACTERIZATION

3-Isopropylmalate dehydrogenase encoded by the Thiobacillus ferrooxida ns leuB gene was purified to homogeneity from Escherichia coli cells h arboring a recombinant plasmid containing the leuB gene. The native en zyme molecule is a dimer of molecular weight 38,000. The K(m) value fo r 3-isopropylmalate was estimated to be 26 muM and that for NAD+ 0.8 m M. The presence of K+ or NH4+ is essential for the enzyme reaction. Th e enzyme is activated about 4-fold by the addition of 1.0 mM Mg2+ or C o2+. The optimum pH and temperature for the activity are 9.0 and 60-de grees-C, respectively. The properties of the enzyme are similar to tho se of the Salmonella typhimurium and Thermus thermophilus enzymes, exc ept for substrate specificity. T. ferrooxidans 3-isopropylmalate dehyd rogenase is able to utilize D- and L-malate as substrates in addition to 3-isopropylmalate. Sequencing of subcloned DNA revealed that the le uB gene consists of a 1,074 bp open reading frame and encodes 358 amin o acid residues corresponding to the subunit (38,462 Da). The amino ac id sequence of 3-isopropylmalate dehydrogenase from T. ferrooxidans an d those of some heterotrophic microorganisms have high homology.